Typically investigations must especially be made of the surfaces of turbine blades. The detection is optical: a radiation pattern is projected onto the surface, than then a picture of the surface is taken. What is here understood by projection of a radiation pattern is that the projected beam has spatial variations in intensity in cross section. One example of a radiation pattern is a strip pattern in which the intensity of the radiation is modulated in sine-wave fashion for example along an axis, and remains constant along the direction perpendicular thereto, corresponding to the value of the sine-wave-modulated intensity. Instead of a strip pattern, a checkerboard pattern can also be used. If the surface is totally flat and the radiation pattern is incident in exactly perpendicular fashion onto the surface, then in the picture recordings one obtains a precise depiction of the emitted radiation pattern. The more the surface deviates from a flat surface, the more the radiation pattern is distorted. Thus, by means of a picture analysis, based on the distortion of the radiation pattern by which structures having their origin in the radiation pattern are formed, retroactive conclusions can be made about the surface curvature. If a suitable computer evaluation unit is part of the arrangement, a three-dimensional picture of the surface can be computer-derived. At regular intervals the radiation pattern is temporally varied, so that the surface can be searched precisely.
The result has been that with optical detection of a surface of numerous objects, for example metallic objects, which engine parts are, disturbing reflections occur. Owing to the reflections, detection of the structures deriving back to the radiation pattern in the pictures becomes difficult, and the place on the surface of the object from which the reflection propagates is only analyzed with difficulty. To reduce reflections, often diffusely scattering layers are applied to the objects to be investigated. However, only certain diffusely scattering layers can be applied on engine parts, because they require authorization. It is time-consuming to apply the layers. They also are applied on the thickness, which impairs the precision of surface analysis. Diffusely scattering layers also cannot suppress intensity noise, often designated as speckle noise. Lastly, diffusely scattering layers must be removed, which is very difficult.